This invention relates generally to electrical contactors and more specifically to improved robustness of small contactors suitable for very high frequency signals.
Integrated circuit chips are tested at least once and sometimes several times during their manufacture. In order to test the integrated circuit chips rapidly, automated handling equipment inserts the chips very rapidly into an automatic test system. The automatic test system includes a device which makes electrical contact to the integrated circuit chip, allowing the automatic test system to generate and measure electrical test signals at the chip being tested.
When the integrated circuit chips are tested after they have been packaged, the device which makes electrical contact to the chips is called a xe2x80x9ccontactor.xe2x80x9d When the integrated circuit chips are tested before they are packaged, the device which makes electrical contact is called a xe2x80x9cprobe card.xe2x80x9d A wafer containing integrated circuit chips is pressed against the probe card by an automated handling device for testing the integrated circuits on the wafer.
A contactor is generally made of conductive beams that provide compliance in one direction. Sometimes, the beams are curled to provide greater spring force in that direction. To make electrical contact, the chip to be tested is pressed against the contactor. Conductive leads or contact pads on the chip make contact with beams in the contactor such that the spring force of the beam causes electrical contact to be made with the chip.
A probe card has some similarities, though the scale is much smaller. A probe card traditionally contains an array of needle-like wires that make contact with pads on the surface of the integrated circuit chip. However, in some instances, probe cards have been made using beams that provide compliance in the direction in which the wafer is moved to press it against the probe card.
There are various limitations with existing contactors for testing packaged parts and probe cards for testing integrated circuit chips on wafers. Probe cards are generally difficult and costly to make. In addition, because the wires or beams used to make the probe cards are so small, they are fragile and can be easily damaged. The probes are designed for compliance only in one direction, specifically the direction in which the wafer is pressed against the probe card. Thus, the probe cards are particularly susceptible to damage from forces that are orthogonal to this direction.
Contactors suffer from similar limitations, but in recent years the limitations have not been as noticeable because contactors are made on a larger scale. The leads on a packaged part have traditionally been spaced much further apart than the pads inside an integrated circuit chip. However, new packaging techniques have reduced the spacing between contact points (or the xe2x80x9cpitchxe2x80x9d) on the packaged parts. For example, one current packaging technique is called Ball Grid Array, or xe2x80x9cBGAxe2x80x9d packaging. A BGA package has an array of pads to which solder balls are attached. To attach the BGA package to a printed circuit board, the solder balls are aligned with conductive pads on the printed circuit board. The solder balls are heated, forming a solder joint between the pad on the chip package and the pad on the printed circuit board.
The pads on a BGA package are typically on a 1.27 mm pitch. Future BGA packages will likely be on a 0.5 mm pitch. Forming beams in a contactor that make good electrical contact with the solder balls requires that the beams have a relatively long aspect ratio. The xe2x80x9cstrokexe2x80x9d or amount of compliance of the beam must be sufficient to ensure that all beams press firmly against all contact points when the BGA Package is pressed against the contactor. For a given width beam, the compliance of the beam will increase as the third power of its length. However, when the pitch of the pads is very small, the length of the beams is limited. Therefore, the beams must be made very narrow and thin to provide the required aspect ratio. Thin, narrow beams are not robust and even more prone to damage. Alternatively, to provide more robustness using known designs, the beams would have to be made too short to provide enough compliance and range of motion to make good, repeatable electrical contact.
Some companies have made contactors with lay-down beams. The beams of these contactors are intended to deflect downwards, towards the substrate to which the beams are attached. However, the beams of these contactors have been so long that they can bend sideways and sometimes do not stay aligned with the contact points on the chips. Additionally, the surface of the substrate below the beams has been made as a conductor. In use, the beam is intended to be pressed down against the conductor to make a low resistance electrical path. However, in practice, it has been observed on occasion that air gaps exist between the beam and the conductor, which can alter the electrical properties of the contact element. It would be desirable to have a contact element with consistent electrical properties even if deflection of the beam is not uniform
Another limitation with existing contactors and probe card designs also relates to the difficulty in making robust beams with very small pitch. In addition to being made smaller, integrated circuit chips are also operating at higher and higher frequency. The test systems that test the chips must test those chips at those high frequencies. However, at high frequencies, the transmission line effects of the connections between the test system and the chip can become a significant source of error. For example, when testing a chip, signals are sometimes measured at a test point and are at other times driven to that same test point. The test system includes, for each test point, a driver and a comparator so that a signal can be either generated or measured. If the driver and comparator are connected to the test point through the same path, the comparator can not operate until the transmission line effects from the driver have faded away. Likewise, the driver can not operate until the comparator has completed measurement, taking into account the transmission line effects. Thus, the transmission line effects limit the time between successive test operations and therefore the speed at which the test system can operate.
One way that has been used to limit the impact of the transmission line effects is called xe2x80x9cfly byxe2x80x9d testing. The drivers and comparators are connected to the test points through completely separate paths. In this way, the transmission line effects triggered by drive signals do not impact measurements made by the comparators. However, making completely separate paths requires two points of contact to each test point on the chip. Thus, the pitch of the contactors would be even smaller than the pitch of the test points, which increases the difficulty in providing a robust mechanical design. As a compromise between the electrical and mechanical properties, the signal paths between each test point and the drivers and comparators is separate for as much as possible, but is not separate in the contactor. Nonetheless, these designs are still limiting for frequencies of 1 GHz or above.
With the foregoing background in mind, it is an object to provide a contactor design that is on a very small scale.
It is also an object to provide a contactor design that is robust.
Further, it is an object to provide a contact design that facilitates fly-by testing.
The foregoing and other objects are achieved with a contact element having at least two conducting members that intersect at a contact point.
In a preferred embodiment, an array of contact elements is disposed on a carrier member with the conducting members interspersed, thereby reducing the pitch between contact points. In one embodiment, the contact point of one contact element is disposed between the conducting members of an adjacent contact element.
In a preferred embodiment, the conducting members of each contact element will be joined at an angle that is less than 90xc2x0.
In a preferred embodiment, one conducting member of each contact element will be connected to a driver and the other conducting member will be connected to a comparator, thereby allowing fly by testing.